I bought a phone off of amazon and they told me it would be unlocked. However, I tried to register the phone to sprint when it came it and it was not compatible. Is there any way I can still use this phone that needs a sim card while having a CDMA service?
A GSM phone will not work on a CDMA system and also a CDMA phone will not work on a GSM system because of the phones hardware. If it was software related then that's a different story and because it's hardware any kind of hack will surely void the warranty and is not guaranteed to work. However it's your phone and you can certainly try if you want to.